Identification of paper media using impedance analysis

Abstract

A microcontroller-based system for identifying a paper type of a sample of paper from a measurement of its electrical impedance. An interdigital dielectric sensor ( 55 ) is deployed in the paper path of a printer (PTR), and the electrical impedance at the sensor, as affected by a sheet of paper (P) near the sensor, is measured over a plurality of frequencies of a stimulus signal. The stimulus signal may be sinusoidal or a square wave. The impedance characteristic, in magnitude or phase, or both, is compared against a plurality of reference impedance characteristics, each associated with a paper type, to identify the closest match and thus the type of paper of the sample sheet.

What is claimed is: 
     
       1. A system for identifying a paper type of a paper sample, the system comprising:
 a memory configured to store reference impedance characteristics of paper types; 
 stimulus generator circuitry having a stimulus output, the stimulus generator circuitry configured to generate a stimulus at the stimulus output; 
 a sensor having: a stimulus component coupled to the stimulus output, the stimulus component configured to generate electric fields through the paper sample at fundamental frequencies responsive to the stimulus; and a sense component configured to sense the electric fields and generate analog signals representative thereof; 
 an analog-to-digital converter having: an analog input coupled to the sense component; and a digital output; and 
 integrated circuitry coupled to the digital output and the memory, the integrated circuitry configured to: receive digital information from the digital output, the digital information representative of the analog signals; responsive to the digital information, determine a sample impedance characteristic for the paper sample; and compare the sample impedance characteristic with the reference impedance characteristics to identify the paper type of the paper sample. 
 
     
     
       2. The system of  claim 1 , wherein the stimulus is a sinusoidal stimulus. 
     
     
       3. The system of  claim 1 , wherein the stimulus is a square wave stimulus. 
     
     
       4. The system of  claim 3 , further comprising:
 clock circuitry configured to generate a base clock signal; and 
 an anti-aliasing filter, coupled between the stimulus output and the stimulus component, the anti-aliasing filter configured to filter the stimulus to remove frequencies above a threshold; 
 the stimulus generator circuitry comprising a digital driver circuit configured to generate the square wave stimulus at a fundamental frequency having a period equal to a first integer number of cycles of the base clock signal; 
 the analog-to-digital converter being configured to receive the analog signals at a sample frequency having a period equal to a second integer number of cycles of the base clock signal; and 
 the integrated circuitry being configured to execute a discrete Fourier transform on the digital information, using a window of samples numbering an integer multiple of a separation number, the separation number defined by: the first integer, divided by a greatest common divisor of the first and second integers. 
 
     
     
       5. The system of  claim 1 , wherein the sensor comprises:
 an interdigital dielectric sensor. 
 
     
     
       6. The system of  claim 1 , wherein the stimulus generator circuitry, the analog-to-digital converter, and the integrated circuitry are integral in a single integrated circuit. 
     
     
       7. The system of  claim 1 , wherein the sample impedance characteristic comprises:
 a sample magnitude versus frequency characteristic; and 
 a sample phase versus frequency characteristic. 
 
     
     
       8. The system of  claim 7 , wherein the integrated circuitry is configured to compare the sample impedance characteristic with the reference impedance characteristics by executing operations comprising:
 detecting a polarity of the first derivative of the sample phase versus frequency characteristic over frequency; and 
 selecting one or more of the reference impedance characteristics exhibiting a phase versus frequency characteristic with a same polarity of the first derivative over frequency. 
 
     
     
       9. The system of  claim 1 , wherein the integrated circuitry is configured to compare the sample impedance characteristic with the reference impedance characteristics by executing operations comprising:
 comparing a value of the sample impedance characteristic at a first frequency with values of at least first and second reference impedance characteristics at the first frequency; 
 determining which of the at least first and second reference impedance characteristics most closely matches the sample impedance characteristic at the first frequency; and 
 repeating the comparing and determining at a second frequency. 
 
     
     
       10. The system of  claim 1 , wherein the integrated circuitry is configured to compare the sample impedance characteristic with the reference impedance characteristics by executing operations comprising:
 applying the sample impedance characteristic to an adaptive network. 
 
     
     
       11. A method of identifying a paper type of a paper sample, the method comprising:
 placing a paper sample near a sensor; 
 applying a stimulus signal at fundamental frequencies to the sensor; 
 sampling a response signal from the sensor, in response to the stimulus signal; 
 processing the sampled response signal for the fundamental frequencies, to determine a sample impedance characteristic for the paper sample; and 
 comparing the sample impedance characteristic with the reference impedance characteristics to identify the paper type of the paper sample. 
 
     
     
       12. The method of  claim 11 , wherein the stimulus signal is a sinusoidal stimulus signal. 
     
     
       13. The method of  claim 11 , wherein the stimulus signal is a square wave stimulus signal. 
     
     
       14. The method of  claim 13 , further comprising:
 generating a base clock signal; and 
 filtering the stimulus signal to remove frequencies above a threshold; 
 wherein applying the stimulus signal comprises generating the square wave stimulus signal at a fundamental frequency having a period equal to a first integer number of cycles of the base clock signal; 
 wherein sampling the response signal comprises sampling the response signal at a sample frequency having a period equal to a second integer number of cycles of the base clock signal; and 
 wherein processing the sampled response signal comprises executing a discrete Fourier transform on the sampled response signal, using a window of samples numbering an integer multiple of a separation number, the separation number defined by: the first integer, divided by a greatest common divisor of the first and second integers. 
 
     
     
       15. The method of  claim 11 , wherein the sensor comprises:
 an interdigital dielectric sensor. 
 
     
     
       16. The method of  claim 11 , wherein the sample impedance characteristic comprises:
 a sample magnitude versus frequency characteristic; and 
 a sample phase versus frequency characteristic. 
 
     
     
       17. The method of  claim 16 , wherein comparing the sample impedance characteristic comprises:
 detecting a polarity of the first derivative of the sample phase versus frequency characteristic over frequency; and 
 selecting one or more of the reference impedance characteristics exhibiting a phase versus frequency characteristic with a same polarity of the first derivative over frequency. 
 
     
     
       18. The method of  claim 11 , wherein comparing the sample impedance characteristic comprises:
 comparing a value of the sample impedance characteristic at a first frequency with values of at least first and second reference impedance characteristics at the first frequency; 
 determining which of the at least first and second reference impedance characteristics most closely matches the sample impedance characteristic at the first frequency; and 
 repeating the comparing and determining at a second frequency.